1. Field of the Invention
The present invention relates to a method for separating hazardous substances from used foundry sand mixtures and is particularly adapted for the separation of heavy metals, such as lead, from used foundry sands so that the heavy metals can be placed in sealed containers for disposal while allowing the recovered sand to be reused in foundry manufacturing processes.
2. Description of the Prior Art
Foundry sand is commonly used to make core molds into which ferrous and non-ferrous metals are cast. The core molds consist of sand bonded with special additives including inorganic binders such as clay, and organic binders, such as phenols, melamine, or urea formaldehyde.
Previously, after the casting has set within the mold, the mold was broken away and discarded. Various factors such as the depletion of natural sand deposits and the cost of disposing of used sand in accordance with recent environmental regulations have now made it economical and advantageous to reclaim the used sand for repeated use.
The type binder used has, in the past, generally dictated the type of reclamation process utilized. Thus, for the inorganic, clay-bonded sands, so called "wet" and "dry" scrubbing techniques have been employed. U.S. Pat. No. 2,261,947, to Barnebl et al, issued Nov. 11, 1941, entitled "Foundry Practice", utilizes the wet scrubbing method in which clay-bonded sands are cleaned in a blasting room by means of high pressure stream of water and sand projected from guns. In the "dry" scrubbing technique, the "dry" sand is projected against an abrasive surface to crack off the clay binder. In both cases, the cleaning action is that of mechanical attrition.
For those sands utilizing the organic resin binders, thermal reclamation techniques have commonly been employed. For example, U.S. Pat. No. 2,478,461, to Connolly issued Aug. 9, 1949, entitled "Apparatus And Method For Treating Foundry Sand", discusses the reclaiming of foundry sand by heating or roasting treatment which causes the organic binders to be burned away. Thermal methods have commonly employed multiple-hearth type furnaces with mechanical "rabble arms" which worked over the used sand and moved the sand through the hearth at elevated temperatures.
None of the previously described techniques proved to be entirely satisfactory for use in reclaiming both the clay-bonded and organic resin-bonded sands, as where a mixture of the sands is present. Also, the efficiency of the previous techniques did not exceed about 70%, with the balance of the waste sand being discarded after each reclamation cycle in municipal landfills.
The majority of the chemical compounds used to bind the aggregate sand grains together to form molds and cores are classified by the U.S. Environmental Protection Agency as air pollutants or as heavy metal, soil and water pollutants. In the case of those foundries casting high leaded bronze, their waste sand contains a high level of lead in addition to the hazardous chemical residues of the resin-bonded molds and cores.
U.S. Pat. No. 4,549,698, issued Oct. 29, 1985, to Andrews, entitled "Method Of Reclaiming Foundry Sand", and assigned to the assignee of the present invention, presented a novel method for reclaiming mixtures of both organic resin-bonded sands and clay-bonded sands. The method utilized a triple fluid bed thermal reactor which combined thermal and abrasive action within the beds to remove both resin-bonded and clay-bonded materials in one continuous operation. The method was not specifically directed to removal of lead contaminated foundry sand, however.
A proposed process for treating a lead containing waste sand to produce a landfill material impervious to the leaching effects of ground or rain waters is described in U.S. Pat. No. 4,408,985, issued Oct. 11, 1983, to Anderson et al. The process described is a "roasting" method using a rotary kiln. It is claimed that a chemical reaction occurs between the oxidized lead and silicon dioxide to form lead silicates, thereby "fixing" the lead as an insoluble compound on the sand grains. However, attempts to substantiate the theory of lead fixation by this method have proved unsuccessful.
The Final Rulings Of The U.S. Environmental Protection Agency, Title 40, Section 132,0102, Mar. 11, 1980, as described in 40 CFR Part 260 et al. include a specific method for determining the leachate in a solid waste. The new method, "Toxicity Characteristic Leaching Procedure" is based on acid digestion of metals. The current maximum permissible lead content in the "leachate" is 5 milligrams per liter. Expressed as a ratio, this is 5 parts of lead per million parts of water. Because the test procedure is based on acid digestion of metals, the procedure effectively eliminates any means of diluting toxic waste remaining in the sand or "fixing" the toxic waste as an insoluble compound and still comply with the regulations. In addition, a standard of 0.51 milligram per liter has been promulgated for treated toxic wastes.
The present invention has as its object to provide a method for separating the useful base aggregates in waste foundry sands in order that they might be re-used in a closed recirculating sand system that functions as an integral part of a foundry's manufacturing processes.
The invention also has as its object to render the base aggregates non-hazardous as a direct result of a physical separation of the hazardous components from the base aggregate.
Another object of the invention is to highly concentrate the hazardous compounds in the waste as a direct result of the separation method.
Another object is to provide a method whereby all hydrocarbons in the off-gas stream are thermally destroyed, all acid compounds in the off-gas stream are neutralized and all particulate in the off-gas is filtered out prior to release to the atmosphere within the threshold limit values (TLV) promulgated by the final rulings of the U.S. Environmental Pollution Agency.
A further object of the invention is the recovery of a national resource and its conservation.
Another object is the reduction of waste material disposal cost and a reduction in foundry manufacturing cost.